The present invention relates to a printing method, particularly to a printing method of printing by an inkjet method on a colored printing medium.
For its simple structure and high-speed printing capability, the inkjet method is widely used, and trials are being made to print on not only paper but a variety of printing media including cloths, textiles, and smooth resin surfaces. Recently, printing is also tried on a colored printing medium having not been sufficiently decolored or a printing medium having been, for instance, deliberately colored, and desired color development is difficult to be achieved.
For instance, when ink is printed on a colored printing medium such as corrugated paper, the color of the printing medium interferes with the color of the ink and it is difficult to achieve the color development of the ink printed on a surface of the printing medium as it is. In particular, a brighter or higher saturation ink color tends to be more interfered with by the color of the printing medium.
To cope with it, pigment ink having light scattering ability is widely used for printing on a colored printing medium. The pigment ink printed on a surface of the printing medium scatters specified type of light and hides the color of the printing medium, which enables the color development of the ink to be achieved as it is without interference from the printing medium color. For example, organic pigment ink or inorganic pigment ink absorbs light at the pigment ink-specific wavelength while reflecting light at other wavelengths by the light scattering ability of particles, and can hamper the absorption of light at the printing medium-specific wavelength. In addition, by printing white particle ink composed of resin particles, inorganic pigments or the like and having high light scattering ability on the printing medium, white particles serve to color a surface of the printing medium white as well as hiding the color of the printing medium. Furthermore, when the white particles having light scattering ability are used in combination with ink having no light scattering ability, the combination works like pigment ink having hiding power.
In order to improve the light scattering ability of the pigment ink printed on the surface of the printing medium to hide the color of the printing medium, however, pigments contained in the ink need to have a large particle size to some extent. Accordingly, when such pigment ink is used in the inkjet method, this causes clogging of nozzles of a print head and therefore requires time-consuming maintenance on the print head.
As a technique to inhibit clogging of nozzles of a print head, a printing method in which white particle ink and an aggregating agent that aggregates the white particle ink are supplied to a colored printing medium is proposed by, for example, JP 2006-45294 A.
In the printing method described in JP 2006-45294 A, the white particle ink containing white particles with a small particle size is supplied to the printing medium by the inkjet method, thereby inhibiting clogging of nozzles of a print head with the white particles. In addition, the white particle ink supplied to the printing medium is aggregated by the aggregating agent and forms aggregates of the white particles so that the particle size is increased, whereby the print in white can be given on the surface of the printing medium with the color of the printing medium being hidden.
However, when pigment ink is used, a resin component serving as a binder needs to be added to the ink in order to fix pigments to a printing medium when the ink is dried up. As a result, when the ink is dried up, the resin is deposited at an unfavorable portion on a print head and therefore, the maintenance of the print head requires a certain level of labor in, for example, removing the deposited resin. Furthermore, pigment ink has a problem in that a color material is expensive. Meanwhile, dye ink in which a water soluble dye is dissolved in an aqueous medium is known as a type of ink which hardly allows clogging of nozzles of a print head to occur and whose color material is inexpensive. However, such dye ink has little light scattering ability and therefore, it has been difficult to achieve the color development of the dye ink as it is due to interference from the color of a printing medium P.